1. Technical Field
The present invention relates generally to a distance measuring apparatus designed to transmit laser pulse signals at given cycles to receive the signal reflected from a reflective object to determine the distance to the object based on a time difference between the transmission of the pulse signal and the reception of the reflected signal. More particularly, the invention is directed to such a distance measuring apparatus which may be employed in an automotive anticollision system which measures the distance to an object present ahead of a vehicle for avoiding a collision therewith.
2. Background Art
Japanese Patent First Publication Nos. 59-142488, 60-201276, and 62-15480 teach conventional distance measuring systems for automotive vehicles which transmit pulse signals in the form of light waves or milimetric waves intermittently, and receive the signal reflected from a reflective object to determine the distance thereto based on a time difference between the transmission of the pulse signal and the reception of the reflected signal.
Specifically, Japanese Patent First Publication No. 59-142488 performs the so-called STC (Sensitivity Time Control) that decreases the sensitivity of signal reception upon transmission of a light signal, and then increases it with time. Usually, when a vehicle travels in a foggy or snowy condition, the system receives light diffused by particles of fog or snow. The reflectivity of fog or snow is much smaller than that of a vehicle body, but the reflection occurs at shorter distance, resulting in increased intensity of a received light (the intensity of reflected light is proportional to the distance to a reflective object to the fourth power). Therefore, the system taught in this publication decreases the sensitivity of reception of light reflected from a short range in order to prevent the distance to an reflective object from being mistakenly determined based on light reflected from fog.
Japanese Patent First Publication No. 60-201276 teaches a system which stores a light intensity range of light reflected from particles such as fog floating in the air relative to the distance to the particles, and provides no output when a signal level of received light falls within the light intensity range. In practice, when the intensity of received light &lt;20 P where P is a light intensity limit in shortest detection range, and the distance L&lt;20 m, the system considers received light as being reflected from fog.
Japanese Patent First Publication No. 62-15480 discloses an automotive fog detection system which judges fog to be formed when a vehicle speed is greater than a given value and derived distance data shows a value more than a preselected value over a given period of time. This is based on the following facts. For example, when a preceding vehicle is traveling at more than a given speed or an obstacle present ahead remains stationary, derived data of distance to the object is changed immediately. In addition, even when a preceding vehicle is traveling at the same speed as that of a system vehicle, the distance to the preceding vehicle is usually changed continuously so that it hardly remains constant over a given period of time, while when the fog is formed within a detection range, the distance data assumes a constant value regardless of the speed of the system vehicle.
The above prior art systems, however, encounter the following drawbacks.
(1) It is impossible to distinguish a lower reflectivity object from fog, so that the systems fail to detect the lower reflectivity object if present in a short range.
(2) The systems fail to detect an object to which the distance is to be measured if they detect fog.
The above discussed publications Nos. 59-142488 and 60-201276 are based on the nature of fog that it is usually generated at short range ahead of a vehicle and shows a lower reflectivity to decrease the sensitivity of signal reception upon transmission of a light signal and then decrease it with time or to output no distance signal when a received light signal falls within the given light intensity range. Thus, these systems cannot distinguish between a lower reflectivity object present at short range and the fog.
Usually, reflectors mounted on vehicles and white vehicle bodies show high reflectivities, while, for instance, persons wearing dark clothes, and dirty or dark vehicles show lower reflectivities. In addition, since portions beneath beds of trucks are generally printed black and are level with a light signal outputted from a distance measuring system, they also show a lower reflectivity.
The fog detection system taught in the publication No. 62-15480 detects the formation of fog based on the fact that derived distance data shows a constant value due to the nature of fog regardless of a vehicle speed. Since this system is, however, designed to detect only one object, once the fog is detected, the system cannot detect another object present away from the detected fog the distance to which is to be measured.